This invention relates to dual-frequency receivers, responsive to both millimeter band radiation and to laser radiation.
Dual-frequency detectors responsive to both microwave and infra-red radiation are found in the target seeking systems of aircraft, both piloted aircraft and guided missiles. To obtain an unrestricted field of view, such sensors are generally located at the nose of the aircraft or missile. The prior art teaches to provide reflector systems to provide directivity and gain for detectors of both wavelengths. Such reflector systems are generally of concave or Cassegrain configuration designed on classical optical principles. To obtain maximum resolution and sensitivity, the aperture of the reflecting system should be as large as possible, and in a missile seeker system generally occupies as much of the missile cross-section as possible. This means that both wavebands have to use the same aperture, and such systems are commonly referred to as common aperture receivers.
In a conventional reflecting system consisting of purely reflecting elements, all wavelengths are brought to focus at the same point. While U.S. Pat. No. 4,282,527 teaches wavelength discrimination by providing a fiber optic cable coaxially with a microwave waveguide at the focus of a reflecting system to guide infra-red radiation and microwave radiation to respective sensors in different locations remote from the focus, the more usual approach is to incorporate one or more dichroic elements in the reflecting system so as to bring different wavelengths to a focus on different detectors at different points in space. Examples of such prior art arrangements are disclosed in U.S. Pat. No. 5,373,302; No. 5,327,149; No. 5,214,438; No. 5,130,718; and No. 3,165,749.
It is also known to provide dual mode detectors where the reflecting system only reflects and focusses infra-red radiation. Microwave radiation is sensed by an array of antennas located either on the surface of a principal infra-red radiation reflector or else behind a principal dichroic reflector which is transparent to microwaves. Such arrangements are disclosed in U.S. Pat. No. 4,477,814 and No. 5,307,077, respectively.
In these prior art arrangements, the component parts of the dual mode detectors, that is to say, the respective detectors responsive to different wavebands and the reflecting systems associated therewith, have been in fixed spatial relationship to each other. Beam steering has involved physically steering the reflecting system and both detectors. This has necessitated the provision of correspondingly robust gimbal arrangements and the need for relatively powerful actuators to overcome the inertia of the reflectors and detectors.